The first great rain in centuries triggers the death wave in the driest desert on earth

After at least 500 years of no significant rainfall, the Chilean Atacama Desert finally rains. Quite unexpectedly, however, these rains – rather than promoting life – do the opposite.

Life on Earth can not exist without water, but for microbes that are highly adapted to dry conditions, the sudden introduction of excess water can be devastating. This is the conclusion of a new paper published in Scientific Reports this week. It is fascinating that, while applicable to life on Earth, these findings can apply to ancient Mars, a planet that may be able to promote microbial life in its ancient past, but at the same time vulnerable for catastrophic flooding.

The 105,000-square-kilometer Atacama Desert, located in northern Chile, is one of the world's oldest and driest deserts. This has been the case for 150 million years. There is a hyperarid nucleus in this desert. Climate models predict larger precipitation events with only one time per century. However, no significant precipitation has been recorded in this region over the last 500 years.

But things change in the Atacama Desert – and not necessarily for the better.

A transient Atacama lagoon. Image: Carlos González Silva

Since 2015, this desert has experienced three major precipitation events – two in 2015 and one in 2017. The water from these rains collected in super-saline lagoons that lasted several months before dispersing. Faced with these unprecedented meteorological events, a team of astrobiologists from Cornell University and the Spanish Center for Astrobiology (CAB) visited Atacama to see how the rain and these hypersaline lagoons affected microbial life in this exceptionally dry location.

Water may be scarce in the Atacama Desert, but that does not mean that the area can not support life. The soil contains a lot of salt, nitrates and sulfates. Organic compounds, not so much. However, it is known that in dry soil a surprisingly large amount of life is available. Organisms that represent all three areas of life (bacteria, archaea and eukaryotes). Extremophile microbes have found a niche after millions of years of extraordinary environment.

The recent rains were, as the new study shows, these tiny creatures not nice. Instead of blooming, the rain has caused enormous devastation to the microbial species that have made the desert their home for millennia.

"Here we show that the sudden and massive water input into regions that have remained hyperarid for millions of years is detrimental to most soil microbial soil osmotic shock when water suddenly becomes plentiful," the authors write in the study.

By "osmotic shock," researchers mean a process in which a sudden change in water concentration interferes with the normal functioning of a cell. It's basically a fancy term for drowning cells.

The unprecedented rains are, according to the authors, the result of changing climatic conditions in the Pacific. From the Pacific came a huge "mass of clouds" into the desert – an "unprecedented phenomenon," say the researchers, who have appeared twice in three years.

The resulting precipitation led to the widespread extinction of many native microbial species. The local extinction rate reached 85 percent, according to the new study in the most affected places. Extremophilic organisms, accustomed to dry conditions, could not cope with the influx of water.

"The hypertrophic soils before the rains were inhabited by up to 16 different old microbial species," said Alberto G. Fairén, Cornell's astrobiologist and co-author of the new study, in a statement. "After it rained, only two to four microbial species were found in the lagoons," said Fairen, who is also a researcher at the Centro de Astrobiología in Madrid. "The extinction was massive."

Among the microbes that survived the liquid attack, a newly identified bacterium was named halomonas,

This study shows that the already low microbial biodiversity that can be found in these extreme environments becomes even smaller when water suddenly appears in large quantities.

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The researchers say that these findings could affect our understanding of how the life of microbes on Mars could have been wiped out if it ever happened there (something we still need to prove). Mars is currently a dry, dusty planet, but it has not always been that way. In addition, it is known that Mars experienced catastrophic flooding in its ancient past.

"Mars had a first period, the Noachian – 4.5 to 3.5 billion years ago – in which there was a lot of water on its surface," said Fairén.

Eventually, the Red Planet lost its atmosphere and its surface water shrank away. However, since this was the case about 3.5 to 3 billion years ago, large amounts of water still flowed to the surface.

"If there were still microbial communities that defy the process of extreme dehydration, they would have been subjected to osmotic processes similar to those we studied in Atacama," said Fairén. "Our Atacama study suggests that the reemergence of liquid water on Mars could have contributed to the disappearance of life on Mars, if it ever existed, rather than creating a chance for resilient microbiota to flower again."

It will be interesting to see what's going on in the Atacama Desert. Will our climate change bring more rain into the desert? And if so, will these rains continue to destroy life or create a new ecosystem? Only time can tell.